1. Field of the Invention
This invention relates to an apparatus for measuring straightness by the utilization of the rectilinear propagative property of a light beam. In the present invention, it is to be understood that the direction of travel of the light beam is the state of straightness and the amount deviating from said direction of travel is straightness. Description will hereinafter be made with an apparatus for measuring straightness utilizing a laser beam taken as an example.
2. Related Background Art
An apparatus using a laser beam in the measurement of straightness has the advantage that it can easily set a reference straight line during the measurement. FIG. 4 of the accompanying drawings schematically shows the construction of an apparatus for measuring straightness according to the prior art.
A laser light source 10 is fixed to a base portion 3. As the light source 10, use is made, for example, of an He-Ne laser. A stage 4 is movable in the direction of arrow S, and the movement thereof is the object of straightness measurement. A light position detector 5 for receiving the laser beam is installed on the stage 4. The light position detector 5 need not always be installed on the stage 4. For example, a reflecting mirror may be installed on the stage and reflected light from this reflecting mirror may be detected by a light position detector.
The origin C is set on the substantially central portion of the light receiving surface of the light position detector 5, and this origin C is used as the reference when the spot position is detected. The light position detector 5 comprises, for example, a four-division photodetector, and when the laser beam is applied to this sensor, a signal conforming to said spot position is sent to a calculating portion 6.
Description will now be made of a case where straightness is measured by the prior-art apparatus shown in FIG. 4. This apparatus measures the spot position on the light receiving surface of the light position detector 5 during the movement of the stage 4 to thereby detect the amount of displacement of this spot position as straightness.
The laser beam emitted from the light source 10 travels through a fluid medium such as air to the light position detector 5 and forms a spot. The light position detector 5 outputs a signal conforming to the position of the spot to the calculating portion 6. The stage 4 is moved by a predetermined distance and the then position of the spot is measured by the light position detector 5. If the stage 4 is moved in a straight direction, the position of said spot will not fluctuate. However, if the stage 4 is moved deviating from a straight direction, the position of said spot will be displaced. The calculating portion 6 finds the amount of displacement of the spot position caused by the movement of the stage 4 from the output value of said spot position output from the light position detector 5. With this amount of displacement as the amount of deviation from the straight direction relative to the direction of movement of the stage 4, straightness d.sub.s has been defined.
The prior-art the apparatus for measuring straightness as described above has suffered from the following problems.
The laser beam propagates through air or some other fluid when straightness is measured. However, the medium density of the fluid tends to cause a fluctuation (hereinafter referred to as the "turbulence") due to the irregularity of the flow or temperature of the fluid. Therefore, the refractive index of the fluid is not uniform but varies in a complicated way. Under such a state, the laser beam does not travel rectilinearly but exhibits deflection. For example, as shown in FIG. 4, the laser beam which is desired to travel rectilinearly as indicated by A and to enter the light position detector 5 actually travels as indicated by B. Also, the direction of said deflection varies with time as the turbulence fluctuates.
As a specific example, in the case of the interior of a room in which no special counter-measure is provided, when the laser beam travels forward by 1 m, there has sometimes been observed an amount of deflection of maximum about 40 .mu.m relative to the straight direction. This value is greater than the order found during the measurement of straightness. Therefore, in an apparatus like the prior-art apparatus in which the spot position on the light position detector is used as the reference, such deflection results in measurement error, significantly reducing the accuracy of the measurement of straightness.